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Cummins/Onan 3A1.7 Engine

The Cummins/Onan 3A1.7 is an engine rated at 1800 rpm. It powers generator sets from Cummins/Onan, including the MDL3. It holds 5.7 L (1.5 gal) of oil and 9.4 L (2.5 gal) of coolant.

Also known as: Cummins 3A1.7

1800 rpm

Rated speed

5.7 L

Oil capacity

On this pageSpecs Generator sets Maintenance FAQ Service intervals Torque & specs Fluids Procedures Wiring

Engine specifications

Oil capacity	5.7 L
Coolant capacity	9.4 L
Rated speed	1800 rpm

Generator sets powered by this engine

Cummins/Onan MDL3

How to read your Cummins/Onan engine serial & model number →

Common problems

No AC output voltage at rated engine RPM
Cause: Control panel field breaker CB21 off
Fix: Push to reset breaker; if voltage is unstable, high or low, or causes breaker to trip, see Flow Charts B, C, D or E
Unstable output voltage, engine speed stable
Cause: Broken wires or loose connections on voltage regulator assembly
Fix: Repair as required
Engine does not crank
Cause: Battery dead
Fix: Check battery per Checkout [A] and recharge or replace; check battery charger operation per Checkout [C]
Engine cranks but does not start
Cause: Fuel supply system problem
Fix: Check fuel tank level, shutoff valves, fuel lines and connections, fuel filters, transfer pump and injection pump; fuel flow should be steady from fuel return line
Output voltage too high or low
Cause: Engine not running at correct RPM
Fix: Set RPM per instructions in appropriate engine manual
Field breaker trips
Cause: Generator leads not connected and grounded properly
Fix: Check generator leads per Test [Q]
Engine starts but stops after running several seconds
Cause: Fault condition indicated by fault breaker CB12 on control panel
Fix: Check for a possible fault condition; if none, check fault monitors S1 through S5 and fault breaker CB12 for improper wiring
Overspeed — fault sensor closes circuit to ground to stop the engine S4
Low oil pressure — fault sensor closes circuit to ground to stop the engine S1
Low coolant level option — fault sensor closes circuit to ground to stop the engine S5
High exhaust temperature — fault sensor closes circuit to ground to stop the engine S3
High coolant temperature — fault sensor closes circuit to ground to stop the engine S2

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan 3A1.7.

Maintenance schedule

Item	Spec	Part #	Interval
Air filter	Check for oil, fuel, cooling and exhaust system leaks. Check exhaust system audibly and visually.	—
Fuel filter		FSl496		Amazon eBay
Oil filter	Spin off the oil filter and discard it. Thoroughly clean filter mounting surface. Apply a thin film of oil to filter gasket, and spin filter on by hand until gasket just touches the mounting pad. Then turn an additional 1/2 turn.	—	500 hr
Battery	Check the terminals on the battery for clean and tight connections. Loose or corroded connections create resistance which can hinder or prevent starting.	—
Belt	Access to the belt is made by removing the belt guard from the front of the genset. Before removing the belt guard, be sure to disable the genset by removing the battery cables-the negative (-) cable first.	—	3 hr
Coolant filter		—
Oil		—
Spark plug		—

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.

Service intervals

Item	Action	Hours	Months	Notes
Valve clearance	Adjust	35 hr		First service — Table 4-1 Note 2: perform after first 35 hours of operation on new sets; Note 4: contact Onan authorized service dealer
Battery electrolyte level	Check	50 hr		Fill cells to bottom of filler neck; add distilled water
Crankcase oil and valve lash	Change	50 hr		First service — Break-in: drain and replace crankcase oil and reset valve lash after first 50 hours on new gensets
Generator bearing	Inspect	1000 hr		Inspect for evidence of outer case rotation
Cooling system	Drain		12 mo	Drain, flush and refill yearly with new coolant
Cooling system pressure cap	Replace		24 mo	Replace every two years or sooner if it malfunctions
Generator bearing	Replace		60 mo	Replace every five years due to grease oxidation

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Service specifications

Type	Item	Value	Unit	Condition
Gap	Overspeed switch magnetic air gap	0.005	inch	minimum
Gap	Overspeed switch magnetic air gap	0.13	mm	minimum
Other	Battery electrolyte specific gravity, fully charged	1.260	specific gravity	80°F (27°C)
Other	Pump belt tension	60	pounds (267 N)	measured with belt tension tool
Pressure	Control power latch S6	5	psi	closes at
Pressure	Cooling system pressure cap rating	7	psi (48 kPa)
Pressure	Engine oil pressure (at operating temperature)	30 to 55	psi (207 to 380 kPa)	engine at operating temperature
Pressure	Low oil pressure fault switch actuation	14	psi (97 kPa)	pressure drops below
Pressure	Low oil pressure switch S7 set point	14	psi	closes at
Pressure	Oil pressure gauge range	0 to 100	psi
Pressure	Oil pressure gauge range	0 to 700	kPa
Resistance	Battery specific gravity, recharge threshold	1.215	specific gravity	charge battery if reading is below
Resistance	Commutating reactor L21, leads 1 and 2	330 to 390	milliohm	77°F (25°C)
Resistance	Exciter rotor ground insulation	1	megohm	minimum; less indicates defective ground insulation
Resistance	Exciter rotor lead pairs T1-T2, T2-T3, T1-T3	486 to 594	milliohms	77°F (25°C)
Resistance	Exciter rotor lead pairs T1-T2, T2-T3, T1-T3	581 to 710	milliohms	77°F (25°C)
Resistance	Exciter stator field leads F1+ to F2-	11 to 14	ohms	77°F (25°C)
Resistance	Exciter stator field leads F1+ to F2-	13 to 16	ohms	77°F (25°C)
Resistance	Exciter stator insulation to laminations (Megger 500 VDC or more)	100,000	ohms	minimum
Resistance	Exciter stator insulation to laminations (ohmmeter)	1	megohm	minimum (1,000,000 ohms)
Resistance	Generator rotor ground insulation	100,000	ohms	minimum
Resistance	Generator stator winding to laminations	less than 100,000	ohms	indicates questionable stator; Megger 500 VDC or more
Resistance	Rectifier bridge CR21 forward resistance	6 to 50	ohms	R x 1 scale
Resistance	Rectifier bridge CR21 reverse resistance	infinity	ohms	R x 10K scale
Resistance	Reference transformer T21, either coil	100 ±10%	ohms	75°F (25°C)
Resistance	Regulating transformer T21 primary/secondary to frame	over 100,000	ohms	Megger or insulation resistance meter
Speed	Overspeed switch S4 trip point, 50 Hz units	1900	r/min ± 90	factory adjusted
Speed	Overspeed switch S4 trip point, 60 Hz units	2200	r/min ± 90	factory adjusted
Speed	Overspeed switch setting, 50 Hz	1800 to 1850	r/min	factory adjusted
Speed	Overspeed switch setting, 60 Hz	2150 to 2200	r/min	factory adjusted
Temperature	Coolant (water) temperature	165 to 195	F (74 to 91 C)	depending on load and ambient temperature
Temperature	Coolant temperature gauge range	100 to 250	°F
Temperature	Coolant temperature gauge range	40 to 121	°C
Temperature	High coolant temperature switch closes	222	F (106 C)	over-temperature
Temperature	High engine temperature switch S2	222	°F	closes at
Temperature	High exhaust temperature switch S3	165	°F	opens again at about
Temperature	High exhaust temperature switch S3	190	°F	closes on temperature rise above
Temperature	High exhaust temperature switch closes	190	F (88 C)	temperature rise above
Temperature	High exhaust temperature switch opens	165	F (74 C)	temperature falls to about
Temperature	Preheat time above 86 F (30 C)	About 10	seconds	ambient above 86 F (30 C)
Temperature	Preheat time below 32 F (0 C)	about 30	seconds	ambient below 32 F (0 C)
Temperature	Preheat time between 50 to 86 F (10 to 30 C)	about 15	seconds	ambient 50 to 86 F (10 to 30 C)
Torque	Drive disk to flywheel	39	ft-lb
Torque	Drive disk to flywheel	52	N·m
Torque	End bell stud nuts	20	ft-lb
Torque	End bell stud nuts	27	N·m
Torque	F1+, F2-, CR1, CR2, CR3, CR4, CR5, CR6 nuts	2.6-2.9	N·m
Torque	F1+, F2-, CR1, CR2, CR3, CR4, CR5, CR6 nuts	23-26	in-lb
Torque	Rotor assembly torque value, Figure 2-9	176-203	N·m
Voltage	AC residual output	35 to 50	VAC	can be
Voltage	AC residual voltage, electronic regulation (generator leads 1 and 2)	5 to 10	VAC	or more
Voltage	AC residual voltage, transformer regulation (leads S1 and S2)	2.5 to 5	VAC	or more
Voltage	Battery cable voltage drop (negative post to cylinder block and positive post to solenoid stud)	less than 0.3	volt	starter motor operating
Voltage	Battery charging voltage, 12-volt system	13.5 to 15	volts	engine running
Voltage	Battery charging voltage, 24-volt system	27 to 30	volts	engine running
Voltage	Generator AC voltage at which starter disconnects	about 90	volts AC	during cranking
Voltage	Solenoid contact voltage drop	1	volt	maximum in circuit application
Voltage	Voltage regulator K16 closing, 12-volt	12.30 to 12.80	volts	70°F (21°C)
Voltage	Voltage regulator K16 closing, 24-volt	24.60 to 25.60	volts	70°F (21°C)
Voltage	Voltage regulator K16 opening, 12-volt	14.25 to 14.75	volts	70°F (21°C)

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Recommended fluids

Fluid	Grade	Standard	Ambient	Capacity	Notes
Coolant	50/50 solution of ethylene glycol anti-freeze and water	Must contain rust and corrosion inhibitor, no stop-leak additive	above -37°C	9.4 L	50/50 mixture provides freeze protection to -34 F (-37 C); do not exceed 50/50; MDL3/MDL4 10 quarts (9.4 L), MDL6 3.3 gal (12.5 L). Use clean, low-mineral, distilled or soft water
Engine oil	15W-40, 30, 20W-20, 10W	API CD/SG, CD/SF or MIL-L-2104C		5.7 L	MDL3/MDL4 capacity 6 quarts (5.7 L); MDL6 capacity 9 quarts (8.5 L). 5W-20 or 5W-30 must be synthetic meeting MIL-L-46167, MIL-L-2104C or MIL-L-46152B; 5W-20/5W-30 mineral base oils not recommended
Fuel	ASTM 2-D (No. 2 Diesel) or ASTM 1-D (No. 1 Diesel)	Minimum cetane number 45; low sulfur, cloud point at least 10 degrees below lowest expected fuel temperature			Use No. 1 diesel when ambient temperatures are below 32 F (0 C) and during long periods of light load
Grease	Molykote				Apply a thin film of Molykote grease to mating surfaces of the end bearing and the end bearing hole.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Procedures

Adjusting the Pump Belt Tension adjustment

⚠ Accidental starting of the generator set can cause severe personal injury or death; disconnect starting battery cables (negative first) before service.

Tools: belt tension tool

Remove battery cables (negative cable first) and remove belt guard.
Inspect belt for ply separation, fraying and glazing; replace if any condition exists.
Measure belt tension with a belt tension tool; should be 60 pounds (267 N).
Loosen bolt through elongated slot in sea water pump bracket and slide pump until proper tension is secured.
Reinstall belt guard; do not operate genset without belt guard in place.

Overspeed Switch Adjustment adjustment

Tools: Accurate tachometer or strobotach

Correct the speed range by turning the adjusting screw, thereby changing the magnetic air gap (Figure 5-3).
Check the overspeed trip point with an accurate tachometer or strobotach after adjustment.

Voltage Adjustment - Electronic Voltage Regulator adjustment

⚠ Do not adjust the other potentiometer (Volts/Hz Adjust); it is factory set using special calibration equipment.

With the generator set running, verify the set has stabilized and is running at correct speed (frequency).
Set the Voltage Adjust potentiometer on the regulator board assembly for correct voltage (Figure 4-12).

Voltage Adjustment - Transformer Voltage Regulator adjustment

⚠ Accidental starting of the set can cause severe personal injury or death. Disconnect the battery cables when repairs are made to the engine, controls, or generator.

Stop the generator set.
Change tap connections on the transformer T21 to adjust output voltage.
When replacement is necessary, install leads to the same terminals of the new transformer.
Refer to the model wiring diagram or the generator reconnection diagrams under Test (Q).

Fuel Solenoid Checkout diagnostic

Watch for solenoid actuation when B+ is applied (start switch in start or run position).
If there is no actuation when B+ is applied, the fuel solenoid must be replaced.
When B+ is removed, the solenoid must de-activate.

Relay Checkout diagnostic

Tools: Voltmeter

Connect relay coil voltage across relay coil terminals; relay should activate if coil is okay.
Connect voltage source to one side of relay contacts.
Connect a voltmeter to the other side of the relay contact and the voltage source.
If voltage appears when relay energizes, contact is okay; reading appears in reverse order when checking normally closed (NC) contacts.

Solenoid Checkout diagnostic

Tools: Voltmeter

Apply battery positive (B+) to the terminal marked S.
Connect a ground wire to the solenoid mounting bracket; solenoid should activate.
If contacts are good, battery voltage should be read between terminal I and ground.
The voltage drop measured across the contacts should never exceed one volt in circuit application.

Switch Checkout diagnostic

Tools: Ohmmeter

Remove battery B+ cable.
Place ohmmeter leads across switch.
Open and close switch while observing the ohmmeter; a normally open (NO) switch should have infinite resistance when open and continuity when closed; a normally closed (NC) switch should have continui
Replace switch if defective.

Testing Rotating Rectifiers diagnostic

Tools: Accurate ohmmeter

Disconnect all leads from assembly to be tested.
Connect one test lead to F1+ stud and other lead to CR1, CR2, and CR3 in turn; record resistance of each rectifier.
Connect one lead to F2- stud and other lead to CR4, CR5, and CR6 in turn; record resistance.
Reverse ohmmeter leads and record resistance of each rectifier again.
All resistance readings should be high in one test and low in the other; if high or low in both, rectifier assembly is defective.
Replace defective rectifier assembly with new identical part.

Wiring Harness Check diagnostic

Tools: Ohmmeter

Inspect all wires for breaks, loose connections, and reversed connections; refer to applicable wiring diagram.
Remove wires from terminals at each end and check each wire end to end for continuity or opens.
Check each wire against each of the other wires and to ground for possible shorts or insulation breaks.
Reconnect or replace wires according to applicable wiring diagram.

Battery Cable Checkout maintenance

Tools: Voltmeter

With the starter motor operating, check the voltage drop from the battery negative post (not the cable clamp) to the cylinder block.
Check the voltage drop from the battery positive post to the battery terminal stud on the solenoid.
Each voltage drop should normally be less than 0.3 volt.
Thoroughly clean all connections in any part of the circuit showing excessive voltage drop.

Battery Checkout maintenance

Tools: Hydrometer

Check charge condition of the battery with a hydrometer.
Electrolyte specific gravity should be about 1.260 for a fully charged battery at 80°F (27°C).
Add distilled water to keep electrolyte at proper level and recharge the battery.
If battery will not recharge, replace it.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Voltage reconnection

Voltage	Phase	Connection	Leads	Notes
		VR21 jumper terminals 6 & 7 for 60 Hz	6, 7	Electronic voltage regulator wiring diagram
		VR21 jumper terminals 7 & 8 for 50 Hz	7, 8	Electronic voltage regulator wiring diagram
110 VAC	1	Transformer regulation, voltage codes 3C, 53C and 538		Move taps on T21 to adjust output voltage
110/190V, 115/220V, 120/208V, 127/220V	3	CODE 518 50HZ		Electronic regulation reconnection
110/220 VAC	1	Transformer regulation, voltage codes 3C, 53C and 538		Move taps on T21 to adjust output voltage
110/220V, 115/230V, 120/240V		CODE 518 50HZ		Electronic regulation reconnection
110/220V, 115/230V, 120/240V	1	CODE 518 50HZ, 1PH 3W		Electronic regulation reconnection
110V, 115V, 120V	1	CODE 518 50HZ, 1PH 2W		Electronic regulation reconnection
120 VAC	1	Transformer regulation, voltage codes 3C, 53C and 538		Move taps on T21 to adjust output voltage
120/208V, 127/220V, 139/240V	3	CODE 18 60HZ		Electronic regulation reconnection
120/240 VAC	1	Transformer regulation, voltage codes 3C, 53C and 538		Move taps on T21 to adjust output voltage
120/240V	1	CODE 18 60HZ		Electronic regulation reconnection
120/240V	1	CODE 18 60HZ, 1PH 3W		Electronic regulation reconnection
220 VAC	1	Transformer regulation, voltage codes 3C, 53C and 538		Move taps on T21 to adjust output voltage
220/380V, 230/400V, 240/416V, 254/440V	3	CODE 518 50HZ		Electronic regulation reconnection
240 VAC	1	Transformer regulation, voltage codes 3C, 53C and 538		Move taps on T21 to adjust output voltage
240/416V, 254/440V, 277/480V	3	CODE 18 60HZ		Electronic regulation reconnection
240V	1	CODE 18 60HZ, 1PH 2W		Electronic regulation reconnection

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Common questions

What is the oil capacity of the Cummins/Onan 3A1.7?

The Cummins/Onan 3A1.7 engine has an oil capacity of 5.7 liters.

Which generators use the Cummins/Onan 3A1.7 engine?

The Cummins/Onan 3A1.7 powers 1 generator set(s), including Cummins/Onan MDL3.

What is a common problem with the Cummins/Onan 3A1.7 engine?

A commonly reported issue on generators using the Cummins/Onan 3A1.7 is: No AC output voltage at rated engine RPM. Cause: Control panel field breaker CB21 off. Fix: Push to reset breaker; if voltage is unstable, high or low, or causes breaker to trip, see Flow Charts B, C, D or E.